Cleavage Process in Frogs

Cleavage is the series of rapid cell divisions that occur after fertilization, forming a multicellular embryo. In frogs, cleavage follows a specific pattern and is crucial to early development.

1.0Cleavage

• Swammerdam observed the first cleavage of a frog's egg in 1738.
• The process of cleavage in a frog's egg was studied by Prevost and Dumas in 1824.
•  In a frog's egg, the cleavage is holoblastic and unequal.
• The cleavage occurs as follows.
• The first cleavage in frog embryogenesis is **meridional**, starting at the animal pole and extending down to the vegetal pole. The second cleavage is also meridional, occurring at a right angle to the first, bisecting it. Both of these cleavages are **holoblastic**, meaning they cut through the entire egg, affecting both blastomeres from the first division and resulting in the formation of four blastomeres.
• The third cleavage is **latitudinal (horizontal)** and occurs above the equator, closer to the animal pole. This positioning is influenced by the yolk concentration in the vegetal pole. The latitudinal furrow divides all blastomeres equally, producing eight cells. Among these, four larger cells, known as **macromeres**, are located in the vegetal pole, while four smaller cells, called **micromeres**, are found nearer the animal pole.
• The fourth cleavage is **meridional and unequal**, dividing the yolk-deficient micromeres more rapidly than the yolk-rich macromeres. This results in a total of 16 blastomeres, with clear differences in size between the micromeres and macromeres.
• The fifth cleavage is latitudinal /horizontal and double, dividing the micromeres and macromeres so that four tiers of blastomeres are formed.
• Blastomeres above the equator are small and remain micromeres; the vegetal pole's blastomeres remain progressively larger. The larger blastomeres are called the macromeres.
• Initially the continued division of blastomeres forms a ball like structure which is solid. It is called the morula stage, as this has superficial resemblance to a mulberry fruit. The morula stage gives rise to a stage called the blastula which is a hollow ball-like structure.

2.0Blastulation

• At the end of cleavage, the solid ball of cells transforms into a blastula composed of numerous blastomeres. A defining feature of this stage is the presence of a distinct cavity known as the **blastocoel**, which represents the beginning of the primary body cavity. The formation of the blastula is referred to as **blastulation**.
• In frogs, the blastula is specifically called an **amphiblastula** because the blastocoel is restricted to the animal pole. In contrast, the vegetal pole consists of a dense mass of non-pigmented, yolk-rich cells. 
• By the 32-cell stage, the blastula takes the form of a single layer of cells, referred to as the **early blastula**. The pigmented micromeres are located in the anterior half of the embryo, while the larger, yolk-laden megameres occupy the posterior half. 
• As already pointed out, the blastocoel lies entirely in the anterior half. 
• The blastula of frogs is hollow and has a very well-developed blastocoel. It is said to be a coeloblastula.
• The floor of the blastocoel is flat, while its top portion is arched. 
• The roof consists of three to four layers of pigmented micromeres, while the floor is formed by yolky metameres. 
• A group of cells intermediate in size (between metameres and micromeres) is found between the micromeres and the metameres and along the equator. 
• These cells constitute the germ ring. The germ ring is formed in the region of the grey crescent.

3.0Gastrulation

• Gastrulation is the process of formation of hollow gastrula from blastula.  
• It involves dynamic movement and rearrangement of blastomeres. Such movements of blastomeres along specific paths during gastrulation are called Gastrulation is the process of formation of a hollow gastrula from a blastula.  
• It involves dynamic movement and rearrangement of blastomeres. Such movements of blastomeres along specific paths during gastrulation are called morphogenetic movements. 
• Three types of morphogenetic movements can be found- invagination, involution and epiboly.
• Invagination: Invagination: Invagination is an active infolding of a blastomere.
• Few blastomeres near the grey crescent are pushed inward to form a slit or groove. 
• The opening of this groove is called blastopore, and the cavity is called gastrocoel or archenteron.
• The blastopore gradually assumes a crescentic shape, eventually becoming circular. The region dorsal to the blastopore opening is called the 'dorsal lip'.
• The lower edge may be called the 'ventral lip'.

• Due to the enlargement of the archenteron, blastocoel is gradually reduced.

4.0Involution

• Involution is the process of rolling in the movement of blastomeres.
• Micromeres multiply and migrate to the dorsal lip of the blastopore, where they roll inside or turn into the archenteron and arrange themselves on its roof.
• Involution is completed by convergence and divergence.
• Micromeres multiply rapidly and move towards the blastoporal end, a process called convergence.
• Thus, converged cells in the blastopore start to involute and slowly diverge towards the roof of the archenteron. This process is called divergence.
• Thus, involuted cells develop into chordamesoderm.
• The archenteron gradually widens, pushing the blastocoel narrow.

• The crescentic blastopore becomes a complete circle.

5.0Epiboly

• Epiboly means the growth of one layer of cells over another.
• Micromeres of the animal pole divide rapidly and move over the macromeres of the vegetal pole. This layer forms an ectoderm.

• As a result, three primary germ layers are formed. The cells which cover the gastrula externally form an ectoderm.
• Those involuted cells in the roof of the archenteron give rise to mesoderm, and cells on the sides and floor of the archenteron develop into endoderm.

• Some other internal changes are also taking place along with those morphogenetic movements.
• As the archenteron enlarges, the yolky megameres are pushed out towards the
• blastopore. This structure is called a yolk plug. 
• The process of gastrulation is completed in 36 hours of fertilization.
• The process of gastrulation converts the blastula into a spherical, bilaterally symmetrical, triploblastic gastrula. Gradually, the gastrula undergoes tubulation or neurulation to become a neurula.